Can I buy a T-mobile pre-paid phone and put my regular T-Mobile SIM card in
it as a backup phone ?

[wsj]
The owner of T-Mobile USA, the fourth-largest U.S. wireless
provider, is exploring an initial public offering or spin
off of the business, according to people familiar with the

T-Mobile has a series of phones that will switch back
and forth between using the t-mobile cellular radio
network and... where available... will send their
signal across an 802.11 ("WIFI") internet connection.